Electrochemical behavior of Fe(III) and mechanism of cathode passivation induced by Fe(III) in MgCl₂-KCl molten salt

Iron impurities severely impair the magnesium electrolysis process by inducing cathode passivation, which inevitably reduces current efficiency and causes irreversible magnesium loss. This study systematically investigates the electrochemical reduction of Fe(III) and its induced cathode passivation mechanism in an anhydrous carnallite (MgCl₂-KCl with a 1:1 molar ratio) melt at 973 K. Through cyclic voltammetry, square wave voltammetry, chronoamperometry, and chronopotentiometry alongside scanning electron microscopy and energy-dispersive X-ray spectroscopy, the results reveal that the reduction of Fe(III) is a single-step, diffusion-controlled quasi-reversible process. Current reversal Chronopotentiometry measurements indicated that the current efficiency substantially decreased from an initial 92.5% to 79.1% after 10 min of electrolysis. Comprehensive morphological and efficiency analyses clearly illuminate the underlying microscopic passivation mechanism. During the initial electrolysis stages, Fe(III) is preferentially reduced and deposits on the cathode surface as a highly porous, sponge-like structure. This newly formed iron matrix readily adsorbs MgO impurities from the molten salt, ultimately generating an insulating Fe-MgO composite film on the electrode. Consequently, this passivation film significantly deteriorates the wettability of the cathode surface to liquid magnesium. It heavily hinders the spreading and agglomeration of magnesium droplets, forcing them to precipitate as fine, discrete, caviar-like particles. Macroscopically, this phenomenon manifests as drastically reduced current efficiency. By elucidating the fundamental electrochemical behavior of Fe(III) and the microscopic mechanics of the Fe-MgO film formation, this study not only enriches essential electrochemical data but also provides a solid theoretical foundation for mitigating cathode passivation in modern industrial magnesium electrolysis processes from a mechanistic perspective.